Soil microbiology guides fertiliser use

Soils were tested for pH, elements including organic carbon and nitrogen, potential nitrification rates and electrical conductivity. Image: USDA NRCSIN AN effort to help improve fertiliser application and timing in agricultural farming, researchers have set out to discover what environmental factors affect the microorganisms in soil and what influence these microorganisms have on nitrification rates.

CSIRO’s Cathryn O’Sullivan says the research highlights soil factors for fertiliser timing so that farmers can apply fertiliser when plants need it most.

“Soils that have high nitrification rates create a run-off from fertiliser that can end up in lakes and rivers and causes a waste of the fertiliser farmers apply to maintain their crops,” Dr O’Sullivan says.

She says oxidation of ammonium into nitrite is performed by two groups of organisms, ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA).

“Until now, there has been little published data on factors which influence the distribution of AOB and AOA in soils and their impact on potential nitrification rates,” Dr O’Sullivan says.

“We wanted to find out which one, or if it was both, contribute to nitrification.

“Through the analysis of agricultural soils in Western and South Australia, we found AOB and AOA were widespread in both states and AOA are particularly hardy, surviving in all conditions.

“We also found their abundance and ratio are mostly affected by the local climate at the time of sampling.”

Researchers collected samples of finer textured South Australian soils in summer when temperatures were higher and rainfall was lower.

Western Australian soil was sampled in winter when the soil was wetter and cooler.

“The South Australian soil samples, collected during dry, hot periods, tended to be AOA-dominated, whereas West Australian samples, collected during cool, wet periods, tended to be AOB-dominated or have an even content of both AOA and AOB,” Dr O’Sullivan says.

Samples were collected from 45 sites within the cropping regions of Western Australia and South Australia.

The study, a collaboration between CSIRO Plant Research and AgResearch at the Lincoln Science Centre in New Zealand, also revealed there was no real correlation between AOA or AOB and potential nitrification rates.

Analysis showed highly fertile soils are likely to experience high nitrification rates regardless of the composition of the ammonia-oxidising microbial community present in the soil.

The results of the study suggest highly fertile soils require management of nitrification to conserve the nitrogen present. Less fertile soils tend to conserve nitrogen due to inherently lower nitrification rates.